A light emitting diode (LED) is a semiconductor device that emits light in a stimulated manner. This effect is a form of electroluminescence. LEDs can provide light in a more efficient manner than an incandescent and/or a fluorescent light source. The relatively high power efficiency associated with LEDs has created an interest in using LEDs to displace conventional light sources in a variety of lighting applications, including for example as traffic lights and for illuminating cell phone keypads and displays.
An LED is comprised of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a pn junction. When biased forwardly, electrons are injected into the junction from the n-region and holes are injected from the p-region. The electrons and holes release energy in the form of photons as they recombine. The wavelength of the light, and therefore its color, depends on the bandgap energy of the materials forming the pn junction.
As semiconductor materials have improved, the efficiency of semiconductor devices has also improved and new wavelength ranges have been used. Gallium nitride (GaN) based light emitters are one of the most promising for a variety of applications, in part because GaN provides efficient illumination in the ultraviolet (UV) to amber spectrum, when alloyed with varying concentrates of indium (In). However, most of the light emitted within a semiconductor LED material is lost due to total internal reflection at the semiconductor-air interface. Typical semiconductor materials have a high index of refraction, and thus, according to Snell's law, most of the light will remain trapped in the materials, thereby decreasing efficiency.
Typically, an LED is made of multiple layers, with at least some of the layers being formed of different materials. In general, the materials and thicknesses selected for the layers determine the wavelength(s) of light emitted by the LED. In addition, the chemical composition of the layers can be selected to try to isolate injected electrical charge carriers into regions (commonly referred to as quantum wells) for relatively efficient conversion to optical power. Generally, the layers on one side of the junction where a quantum well is grown are doped with donor atoms that result in high electron concentration (such layers are commonly referred to as n-type layers), and the layers on the opposite side are doped with acceptor atoms that result in a relatively high hole concentration (such layers are commonly referred to as p-type layers).
Typically, when preparing the LED, the layers of material are prepared in the form of a wafer. These layers are formed using an epitaxial deposition technique, such as metal-organic chemical vapor deposition (MOCVD), with the initially deposited layer being formed on a growth substrate. The layers are then exposed to various etching and metallization techniques to form contacts for electrical current injection, and the wafer is subsequently sectioned into individual LED chips. Usually, the LED chips are packaged.
During use, electrical energy is usually injected into an LED and then converted into electromagnetic radiation (light), some of which is extracted from the LED. A number of references are devoted to the issue of light extraction from light-emitting semiconductor materials. Light extraction can be achieved using geometrical optics effects, for example, using pyramids, outcoupling tapers or textured surfaces, or wave optics effects, for example, using microcavity resonances or photonic crystal extraction. Special growth techniques such as pendeo or cantilever growths or lateral epitaxial overgrowth may also be used to achieve light extraction. However, there have been few recent advances in the field of LED extraction.
Accordingly, there is a need in the art for new light-emitting devices, and related components, systems and methods, and in particular there is a great need in the art for more efficient LEDs and methods for making them.